Outside of the continuity, I'm sure it was added because it was visually cool and to establish that Han (while awesome) was in a completely different league than Vader. It was just another way to show how bad Vader was.
Within the context of the story, I think it was probably the force. If it had been armor, the blast would have burned through the outer glove, which I don't think it did. I think it was the force, and likely something that only came from the Chosen One.
Again, though, the real reason it was added is because it was just some cool **** to see.